Local functions

Here is an example of two functions that are local to the enclosing method:

def average(in: Int*): Int = {
 def sum(in: Int*): Int = in.sum
 def count(in: Int*): Int = in.size
 sum(in:_*)/count(in:_*)
}

In this example, we have both sum and count defined inside of the average definition, which makes them inaccessible from the outside:

scala> :type sum
       ^
       error: not found: value sum

scala> :type count
       ^
       error: not found: value count

As already mentioned, the function does not need to be nested in another method. The enclosing block can be of any kind, for example, a variable definition. For instance, consider if the average function from the previous example was only defined in order to calculate a single average:

val items = Seq(1,2,3,4,5)
val avg = average(items:_*)

We could rewrite both code blocks as follows:

val items = Seq(1,2,3,4,5)
val avg = {
  def sum(in: Int*): Int = in.sum
  def count(in: Int*): Int = in.size
  sum(items:_*)/count(items:_*)
}

The scope visibility rules apply for methods the same way as for other language constructs. Because of this, the parameters of the outer method are visible to the inner functions and don't need to be passed explicitly. We can rewrite our first example again using this rule, as follows:

def averageNoPassing(in: Int*): Int = {
  def sum: Int = in.sum
  def count: Int = in.size
  sum /count
}

There is a special name for the functions that refer to the definitions from the enclosing block, closures. Let's discuss them a little more deeply.